A prior art filter is described in an article published in IEEE MTT Vol. 21 No. 1, January 1973, entitled "Design of evanescent mode waveguide diplexers". This article describes a new procedure for designing filters made up from waveguides below the cut-off. This design enables an entire diplexer to be constructed (with the entire device being constructed as a single waveguide, thereby making it possible to omit a T-junction and connection flanges), providing a coaxial link is used at the common junction. The design described uses bandpass filters which are suitable for bandwidths of a few percent.
With this type of diplexer, the tuning of the two filters is not independent and each filter interacts on the other. The tuning bands of the filters must necessarily be very close to each other since the two filters share a common resonator. In addition, the bandwidth cannot exceed a few percent. Further, this diplexer is necessarily constituted by two in-line filters.
The object of the invention is to make the filters independent from each other, i.e. such that there is no interaction between the filters when one of them is adjusted, thereby simplifying adjustment.